The present invention relates to an optical imaging system for an electrophotographic copying machine and more particularly to an exposure system in which the reflectivity of a scan mirror in the optical system controls the illumination level and illumination profile at the surface of a photoreceptor.
In electrophotographic copiers, a document to be reproduced is placed on a document platen and is incrementally illuminated by a light source such as a fluorescent lamp, which moves in a parallel path beneath the platen. The light is reflected by at least a scanning mirror and the reflected light image projected through a lens to expose a surface of a photosensitive image medium such as a drum or belt photoreceptor. The optical system generally incorporates one or more folding mirrors to enable a compact design. The surface of the photoreceptor has previously received a charge of a certain magnitude and polarity. The incident light discharges the surface leaving fully charged areas representing the informational areas of the document (write-white system). The latent image of the document thus formed is subsequently developed and transferred to an output medium such as plain paper. The goal in the production of the output print for a 1:1 copy is to make as exact a copy of the original document as possible. It is, therefore, desirable that the non-image areas of the photoreceptor which are exposed by the light experience a discharge to the same background level, e.g., achieve a uniform discharge level in all the illuminated areas.
The major source of non-uniform discharge is a non-uniform illumination level incident at the photoreceptor surface. The causes of non-uniform illumination are well known; the most significant being the fall-off in light at the ends of the image plane illumination profile caused by cos.sup.4 variations created by the projection lens. Other causes of non-uniformity include the dark regions near the electrodes in a low pressure gas discharge, such as a fluorescent lamp. In a linear tungsten lamp, the unlighted sections between filiment sections also cause illumination non-uniformities. Lamp aging or deterioration effects during continuing operation also cause illumination non-uniformities along the lamp. Further causes of undesirable non-uniformities are contamination of the optical system by dirt and dust build-up on the lenses and mirror components. Various techniques have been employed in the prior art to compensate for these factors. The cos.sup.4 fall-off is typically compensated for in scanning type systems by imaging light through a butterfly slit positioned adjacent to the photoreceptor surface, or through a slit integral with the lens. Other systems place a variable density filter into the light path, the filter designed to vary the transmission therethrough so as to provide for a uniform level of illumination to irradiate the photoreceptor surface. U.S. Pat. No. 4,298,275 describes characteristics of this type of filter. The effects of lamp aging are compensated for by, for example, the procedures described in U.S. Pat. No. 3,947,117. This patent discloses a detection of a portion of the illumination output level at a photoreceptor and generates a photosensor output which is used to vary the power to the illumination source. This technique changes the overall illumination and does not compensate for variation in illumination profile. Still other systems use specially shaped or positioned reflectors to compensate for lamp uniformity variations. These techniques introduce permanent corrections that can not compensate for dynamic changes in the profile.
Even after the above effects have been compensated for and a uniform irradiance level is obtained at the photoreceptor, yet the discharge of the illuminated portion of the photoreceptor surface may not be completely uniform. This additional non-uniformity is due to factors such as variations in the photosensitivity of the photoreceptor being used (which results in some areas discharging more or less than the other areas even through subject to the same light level), or variations in the initial "uniform" charge applied to the photoreceptor. This second group of factors are more subtle and difficult to compensate for than the non-uniform light profile factors. The present invention is directed towards modifying the optical system of a scanning type of copier by replacing an already existing mirror, or introducing a new mirror in an appropriate location in the optical path, the mirror being constructed of segments of an electrically sensitive material whose reflectance characteristics can be altered by applications of appropriate voltage. In a preferred embodiment, an electrochromic material that changes transmission in response to the applied voltage is overcoated on a mirror surface. Thus, the mirror functions as a segmented variable reflectance mirror in response to an applied voltage. The voltages applied to the mirror segments are derived in a first embodiment from a segmented electrometer which is positioned along the full width of the photoreceptor surface. The electrometer measures charge variations in discrete segmented exposed areas across the width of the photoreceptor. Each segment of the electrometer corresponds to one of the mirror segments. The variations in the charge levels as detected by the electrometer are compensated for by varying the reflectance of the associated mirror segments so as to vary the illumination profile. Variable reflectance non-segmented mirrors are known for other purposes, e.g., U.S. Pat. No. 4,603,946 and a publication High Technology Magazine, pg. 7, published July 1987. Both references disclose changing the reflectance of an auto rear view mirror for changes in ambient conditions.
More particularly, the invention related to a document imaging system wherein a document on a platen is scan/illuminated by a scan mirror and illumination lamp assembly and a document image is projected along an optical path to expose a charged surface of a photoreceptor resulting in a first charge level representing the document information and a second charge level representing exposed background areas;
an exposure control system comprising in combination an electrometer positioned adjacent the surface of said photoreceptor substantially extending along its width, said electrometer comprising a plurality of segmented probes, each probe adapted to sense the background charge level of a discrete area of the photoreceptor surface lying therebeneath and to generate an output signal representative of the charge level of said discrete area,
a variable reflectance mirror fixedly positioned along said optical path, said mirror having a plurality of segments whose reflectance varies in accordance with a voltage applied thereto, and
a control circuit for comparing the output signals from said electrometer segments with a reference signal representative of an optimum or uniform discharge level, and for applying output signals at appropriate voltage levels to corresponding mirror segments whereby the reflectivity of the addressed mirror segment is selectively increased or decreased to cause a decrease or increase, respectively, in said discrete background areas.
For certain systems adequate performance may be achieved by compensating for the light non-uniformities of the light source at the image plane surface. According to a second embodiment of the invention a linear array of photodetectors (rather than the electrometer) is located along the photoreceptor width at the surface. Signals from each individual photosensor are then processed and sent to control the reflectivity of the corresponding mirror segments.